Currently, new processes win recognition in the field of visual display and lighting technology. It will be possible to manufacture flat displays or illuminated surfaces having a thickness of less than 0.5 mm. These are notable for many fascinating properties. For example, it will be possible to achieve illuminated surfaces in the form of wallpaper with very low energy consumption. It is also of particular interest that color visual display units will be producible with hitherto unachievable colorfastness, brightness and viewing angle independence, with low weight and with very low power consumption. It will be possible to configure the visual display units as micro-displays or large visual display units of several square meters in area in rigid form or flexibly, or else as transmission or reflection displays. In addition, it will be possible to use simple and cost-saving production processes such as screen printing or inkjet printing. This will enable very inexpensive manufacture compared to conventional flat visual display units. This new technology is based on the principle of the OLEDs, the organic light-emitting diodes. Furthermore, through the use of specific organometallic materials (molecules), many new optoelectronic applications are on the horizon, for example in the field of organic solar cells, organic field-effect transistors, organic photodiodes, etc.
Particularly for the OLED sector, it is apparent that such devices are already now of economic significance, since mass production of OLED displays for mobile phones has already started. Such OLEDs consist predominantly of organic layers, which can also be manufactured flexibly and inexpensively. Worth pointing out is that OLED components can be configured with large areas as illumination bodies, but also in small form as pixels for displays.
Compared to conventional technologies, for instance liquid-crystal displays (LCDs), plasma displays or cathode ray tubes (CRTs), OLEDs have numerous advantages, such as a low operating voltage of a few volts, a thin structure of only a few hundred nm, high-efficient self-illuminating pixels, high contrast and good resolution, and the possibility of representing all colors. In addition, in an OLED, light is produced directly upon application of electrical voltage, rather than merely being modulated.
A review of the function of OLEDs can be found, for example, in H. Yersin, Top. Curr. Chem. 2004, 241, 1 and H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials”; Wiley-VCH, Weinheim, Germany, 2008.
Since the first reports regarding OLEDs (see, for example, Tang et al., Appl. Phys. Lett. 1987, 51, 913), these devices have been developed further particularly with regard to the emitter materials used, and particular interest has been attracted in the last few years by what are called triplet emitters or by other phosphorescent emitters.
OLEDs are generally implemented in layer structures. For better understanding, FIG. 1 shows a basic structure of an OLED. Owing to the application of external voltage to a transparent indium tin oxide (ITO) anode and a thin metal cathode, the anode injects positive holes, and the cathode negative electrons. These differently charged charge carriers pass through intermediate layers, which may also consist of hole or electron blocking layers not shown here, into the emission layer. The oppositely charged charge carriers meet therein at or close to doped emitter molecules, and recombine. The emitter molecules are generally incorporated into matrix molecules or polymer matrices (in, for example, 2 to 10% by weight), the matrix materials being selected so as also to enable hole and electron transport. The recombination gives rise to excitons (=excited states), which transfer their excess energy to the respective electroluminescent compound. This electroluminescent compound can then be converted to a particular electronic excited state, which is then converted very substantially and with substantial avoidance of radiationless deactivation processes to the corresponding ground state by emission of light.
With a few exceptions, the electronic excited state, which can also be formed by energy transfer from a suitable precursor exciton, is either a singlet or triplet state, consisting of three sub-states. Since the two states are generally occupied in a ratio of 1:3 on the basis of spin statistics, the result is that the emission from the singlet state, which is referred to as fluorescence, leads to maximum emission of only 25% of the excitons produced. In contrast, triplet emission, which is referred to as phosphorescence, exploits and converts all excitons and emits them as light (triplet harvesting) such that the internal quantum yield in this case can reach the value of 100%, provided that the additionally excited singlet state, which is above the triplet state in terms of energy, relaxes fully to the triplet state (intersystem crossing, ISC), and radiationless competing processes remain insignificant. Thus, triplet emitters, according to the current state of the art, are more efficient electroluminophores and are better suitable for ensuring a high light yield in an organic light-emitting diode.
The triplet emitters suitable for triplet harvesting transition metal complexes are generally used in which the metal is selected from the third period of the transition metals. This predominantly involves very expensive noble metals such as iridium, platinum and also gold (see also H. Yersin, Top. Curr. Chem. 2004, 241, 1 and M. A. Baldo, D. F. O'Brien, M. E. Thompson, S. R. Forrest, Phys. Rev. B 1999, 60, 14422). The prime reason for this is the high spin-orbit-coupling (SOC) of noble metal central ions (SOC constants Ir(III): ≈4000 cm−1; Pt(II): ≈4500 cm−1; Au(I): ≈5100 cm−1; Ref.: S. L. Murov, J. Carmicheal, G. L. Hug, Handbook of Photochemistry, 2nd Edition, Marcel Dekker, New York 1993, p. 338 ff). Due to this quantum mechanical characteristic, the triplet-singlet transition, which is without SOC strictly forbidden for optical transitions, is allowed and an emission decay time of a few μs, small enough for OLED applications, is achieved.
Economically, it would be highly advantageous to replace the expensive noble metals with less expensive metals. Moreover, a large number of OLED emitter materials known to date are ecologically problematic, so that the use of less toxic materials is desirable. Copper(I) complexes are to be considered for this, for example. However, these have much smaller SOC values (SOC constants of Cu(I): ≈850 cm−1, Ref.: S. L. Murov, J. Carmicheal, G. L. Hug, Handbook of Photochemistry, 2nd Edition, Marcel Dekker, New York 1993, p. 338 ff) than the central ions mentioned above. Therefore, the very important triplet-singlet-transitions of Cu(I)-complexes would be relatively strongly forbidden, and emission lifetimes, which are in the range of a few 100 μs to ms, would be too long for use in OLEDs. Such high emission decay times give rise to saturation effects with increasing current densities and the resulting occupation of a majority or of all emitter molecules. Consequently, further charge carrier streams can no longer lead completely to the occupation of the excited and emitting states. The result is then more unwanted ohmic losses. This leads to a distinct decline in efficiency of the OLED device with rising current density (called “roll-off” behavior). The effects of triplet-triplet annihilation and of self-quenching are similarly unfavorable (see, for example, H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials”, Wiley-VCH, Weinheim 2008 and S. R. Forrest et al., Phys. Rev. B 2008, 77, 235215). For instance, disadvantages are found particularly in the case of use of such emitters for OLED illuminations where a high luminance, for example of more than 1000 cd/m2, is required (cf.: J. Kido et al. Jap. J. Appl. Phys. 2007, 46, L10). Furthermore, molecules in electronically excited states are frequently more chemically reactive than in ground states so that the likelihood of unwanted chemical reactions increases with the length of the emission lifetime. The occurrence of such unwanted chemical reactions has a negative effect on the lifetime of the device.
Furthermore, Cu(I)-complexes generally undergo strong geometry changes after the excitation (through electron-hole recombination or through optical excitation) which leads to the reduction of emission quantum yields. Also, the emission colors are shifted due to these processes towards red, which is unwanted.
It was the object of the present invention to provide new materials that do not exhibit the disadvantages described above.